Nanoparticle-alginate gels for x-ray imaging of the vasculature

ABSTRACT

Disclosed are capped nanoparticles that are effectively trapped within an aqueous gelling solution to produce stable gels and function as a contrast agent for vascular imaging. The contrast agent has good radioopacity, is inexpensive to produce, and is safe to handle. This provides a new method to image the fine vasculature of biological systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/990,768, filed on Mar. 17, 2020, which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE DISCLOSURE

The present disclosure relates generally vascular imaging. Inparticular, the present disclosure is directed tonanoparticle-containing composite gels for improved imaging thevasculature of biological systems using X-ray techniques.

Vascular imaging is a very important technique to distinguish thevascular network with similar or low X-ray attenuation. The contrastagent is critical for this technique. Unstable contrast agent can easilydissociate or leach from the host matrices, which can lead to fuzzyimaging and misdiagnosis. Also, the lack of inherent radiopaque agentshas severely hindered the imaging technique.

Barium sulfate is a commonly used contrast agent clinically forgastrointestinal imaging and is much more radiodense than bone. However,commercially available barium sulfate particles are already approachingthe size of capillaries and tend to bind in solution to make even largerobjects. Thus, the particles clog at the capillary level, never enterthe venous system, and the pressure ultimately ruptures the aorta.

Accordingly, there exists a need for new and alternative contrast agentsfor imaging vasculature, particularly vasculature within and inproximity to bone.

BRIEF DESCRIPTION OF THE DISCLOSURE

In one aspect, the present disclosure is directed to a contrast agentcomprising a nanoparticle, the nanoparticle comprising at least one of abarium sulfate (BaSO₄) core, a barium carbonate (BaCO₃) core, a calciumcarbonate (CaCO₃) core; and a capping agent comprising at least one ofan oligomer and a polymer; and a gel precursor solution.

In one aspect, the present disclosure is directed to a method forimaging vasculature of a subject, the method comprising: introducinginto the subject's vasculature a contrast agent, the contrast agentcomprising a nanoparticle, the nanoparticle comprising at least one of abarium sulfate (BaSO₄) core, a barium carbonate (BaCO₃) core, a calciumcarbonate (CaCO₃) core; and a capping agent comprising at least one ofan oligomer and a polymer; and a gel precursor solution, thenanoparticle dispersed in the gel precursor solution; crosslinking thegel precursor solution to produce a gel, wherein the nanoparticle isdispersed in the gel; and imaging the subject using an X-ray technique.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be better understood, and features, aspects andadvantages other than those set forth above will become apparent whenconsideration is given to the following detailed description thereof.Such detailed description makes reference to the following drawings,wherein:

FIGS. 1A-1D depict the gelation process. FIG. 1A is a photograph ofpolyethylene-glycol barium sulfate nanoparticles (PBNPs) powder. FIG. 1Bis a photograph of 2% sodium-alginate (Na-Alginate) solution. FIG. 1C isa photograph of PBNPs in water. FIG. 1D is a photograph of PBNPs in 2%Na-Alginate solution.

FIG. 2 depicts the proposed mechanism of crosslinking of Na-Alginatewith Ba²⁺ or Ca²⁺ cations as explained by the “Egg box model”.

FIG. 3 depicts a PXRD pattern of BaSO₄ nanoparticles capped with TEG(crystallite cores=31 nm in diameter).

FIG. 4 is a transmission electron micrograph of BaSO₄ nanoparticlescapped with TEG.

FIG. 5 depicts a DLS measurement of BaSO₄ nanoparticles capped with TEGand shows an agglomerated particle size of 50 nm in diameter.

FIG. 6 is a photograph of dry BaSO₄@TEG powder.

FIGS. 7A & 7B are photographs of a beaker with a solution of BaSO₄nanoparticles in TEG showing well dispersed homogenous BaSO₄nanoparticles in TEG (FIG. 7A) and a concentrated mixture of homogenousBaSO₄ nanoparticles in TEG (FIG. 7B).

FIG. 8 depicts a PXRD pattern of CaCO₃ particles capped with TEG(crystallite cores=41.4 nm in diameter),

FIG. 9 depicts a DLS measurement of CaCO₃ particles capped with TEG andshows an agglomerated particle size of 164 nm in diameter.

FIG. 10 depicts a BaSO₄@TEG, Na-Alginate, Polyacrylic acid sodium salt(PAA), CaCO₃ and GDL composite gel.

FIG. 11 depicts a BaSO₄@TEG, Na-Alginate, Polyvinyl alcohol (PVA) sodiumsalt (PAA), CaCO₃ and GDL composite gel.

FIG. 12 is an X-ray image of a 1% (wt/wt) gel with 0.05 g/mL, 0.1 g/mL,and 0.25 g/mL of BaSO₄ nanoparticles in comparison with bone.

FIG. 13 is an X-ray image of a 2% (wt/wt) gel with 0.5 g/mL of BaSO₄nanoparticles in comparison with bone.

DETAILED DESCRIPTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the disclosure belongs.

In one aspect, the present disclosure is directed to an injectablecontrast agent. The contrast agent includes a nanoparticle core cappedwith a capping agent; and a gel precursor solution.

Suitable nanoparticle core materials include a barium sulfate (BaSO₄)core, a barium carbonate (BaCO₃) core, and a calcium carbonate (CaCO₃)core, and combinations thereof.

Suitable capping agents for capping the core material include oligomers,polymers, and combinations thereof. Suitable polymers includepolyethylene glycol (PEG) including PEG-600 to PEG-500,000. Othersuitable polymers include: polyvinyl alcohol (PVA), polyacrylic acid(PAA), polyacrylamide (PAM), poly(sodium 4-styrenesulfonate) (PSS), forexample. Chain-end modified PEG can also be used such as, for example,those with amino NH₂ groups and hydroxyl OH groups. Low molecular weightoligoethers (oligomers related to PEG) include triethethylene glycol andtetraethylene glycol are also suitable capping agents.

The nanoparticle core is capped with a capping agent includingoligomers, polymers, and combinations thereof as described herein. Asused herein, “capping” and “capped” by the oligomer and polymer cappingagents provides a homogenous but semi-permeable layer, tightly boundaround the ionic nanoparticle core. The underlying ionic structure isthermodynamically stable, but capping imparts favorable characteristicsto the nanoparticle core such as permitting dispersion in aqueous media.Without being bound by theory, the capping oligomer and polymer binds tothe nanoparticle core surface via predominantly ion-dipole interactions.

The gel precursor solution includes a material that forms a gel (a “gelforming material”) upon crosslinking. So the contrast agent isinjectable, the gel precursor solution provides a matrix that carriesthe oligomer- and polymer-capped core nanoparticles to a local tissuesite in the subject. Upon gelation, the gel precursor solution forms agel and retains the nanoparticles in the local tissue site. Suitable gelforming materials include alginate. Other suitable gelling agentsinclude that do not require heating above 50° C., for example, chitosan,silicon-based sol-gels, and gellan gum. Suitably, the amount of alginatein the final alginate composition of the resulting contrast agent canrange from about 0.05% (wt/wt) to about 5% (wt/wt). Preferably, theamount of alginate can range from about 0.1% (wt/wt) to about 2%(wt/wt).

In addition to contrast agents where all of the core particles areprepared using the same core material and have the same capping agent,different combinations of materials are also contemplated herein.Different combinations of materials include combinations of differentcore materials (e.g., BaCO₃ cores, BaSO₄ cores, CaCO₃ cores) with thesame or different capping oligomers and capping polymers (e.g., PEG,TEG). For example, BaCO₃ cores capped with PEG can be combined withBaCO₃ cores capped with TEG, which are then combined with the gelprecursor solution to prepare the contrast agent. Similarly, BaCO₃ corescapped with PEG can be combined with BaSO₄ cores capped with PEG, whichare then combined with the gel precursor solution to prepare thecontrast agent. BaCO₃ cores capped with PEG can be combined with BaCO₃cores capped with TEG and combined with CaCO₃ cores capped with TEG,which are then combined with the gel precursor solution to prepare thecontrast agent. Thus, exemplary contrast agents include BaSO₄-PEG,BaSO₄-TEG, BaCO₃-PEG, BaCO₃-TEG, CaCO₃-PEG, CaCO₃-TEG,BaSO₄-PEG/TEG+/−BaCO₃-PEG/TEG+/−CaCO₃-PEG/TEG, andBaCO₃-PEG/TEG+/−CaCO₃-PEG/TEG.

Suitably, the amount of nanoparticle core in the contrast agent canrange from about 0.05 g/mL to about 1 g/mL. Suitably, the diameter ofthe core particles ranges from about 10 nm to about 500 nm. Particlesize can be determined by methods known to one skilled in the art suchas by Dynamic light scattering (DLS), powder X-ray diffraction (PXRD),transmission electron microscopy, and combinations thereof, for example.

The nanoparticle cores are capped with a capping agent resulting in theformation of the core-shell nanoparticles. As discussed herein, cappingof the core particles with a capping agent stabilizes the core and alsoconfers water dispersibility to the system. The amount of capping agentused in the solution for capping ranges from about 0.05% (w/w) to about30% (w/w). A particularly suitable range of PEG is a range of about 2%(w/w) to about 25% (w/w). A particularly suitable range of TEG is arange of about 0.05% (w/w) to about 30% (w/w), including a range ofabout 0.05% (w/w) to about 10% (w/w). A particularly suitable amount ofPEG is about 2% (w/w). An aqueous capping agent solution is used tosynthesize the nanoparticle composites. The amount of capping agent forthe capped cores can be determined by elemental analysis.

The contrast agent can further include at least a second polymer toimprove gel stiffness. Suitable second polymers include polyacrylic acid(PAA), polyvinyl alcohol, chitosan, and combinations thereof.

The contrast agent can further include a chelating gelation controlreagent for controlling gel formation of the contrast agent. Suitablechelating gelation control reagents include ethylenediaminetetraaceticacid (EDTA) and/or glucono-δ-lactone (GDL) in combination with a calciumcarbonate. Other suitable chelating gelation control reagents include:nitrilotriacetic acid (NTA), trans-1,2-diaminocylcohexanetetraaceticacid (DCTA), diethlyenetriaminepentaacetic acid (DTPA), andbis(aminoethyl)glycolether-N,N,N′,N′-tetraacetic acid (EGTA).

The polymer- or oligomer-capped core nanoparticles are effectivelytrapped within a rigid gel that forms from an aqueous solution bycrosslinking Ca²⁺ ions with gelling polymers such as alginate.Alternatively, Cu²⁺, Zn²⁺, or Mg²⁺ ions may be used as a crosslinker inplace of Ca²⁺.

In one aspect, the present disclosure is directed to a method forimaging vasculature of a subject. The method includes: introducing intothe subject's vasculature a contrast agent, the contrast agentcomprising a nanoparticle, the nanoparticle comprising a core cappedwith a capping agents; and a gel precursor solution, the nanoparticledispersed in the gel precursor solution; crosslinking the gel precursorsolution to produce a gel, wherein the nanoparticle is dispersed in thegel; and imaging the subject using an X-ray technique.

Suitable materials for the nanoparticle core are described herein.

Suitable capping agents for capping the core are described herein.

Suitable gel forming materials are described herein.

Suitable subjects include animals. Particularly suitable subjects arecadavers, primates, mice, rats, rabbits, birds, and other animals,particularly laboratory research animals. Suitable subjects can beanimals suffering from a disease or disorder that would require the useof the contrast agent to image a tissue site. Introducing the contrastagent into the subject's vasculature is through perfusion, particularlypostmortem perfusion.

The gelation rate of the contrast agent can be controlled. Gelation rateis controlled to allow sufficient time for the contrast agent to beintroduced into the vasculature of the subject before crosslinking intoa gel. Thus, the method can further include adding a chelating gelationcontrol reagent for controlling gelation time to one of the crosslinkerand the gel precursor solution. Gelation rates can range from about 10seconds to about 48 hours at a temperature ranging from about 5° C. toabout 40° C. and within a pH ranging from about 5.0 to about 7,including from about 5.5 to about 6.5. Gelation time can be controlledby adding a 0.1 M solution of EDTA to the gel precursor solution, forexample. A suitable alternative to EDTA is calcium carbonate incombination with glucono-δ-lactone. The ratio of Ca²⁺ ions to GDL can beused to control gelation time. In an exemplary gel composition theamounts include 0.5% w/w CaCO₃ and 1.5% w/w GDL (the mass % for thewhole gel including CaCO₃, GDL, Na-alginate, PEG-capped BaSO₄nanoparticles, and also water). A suitable range is from about 0.1% w/wto about 1.0% w/w CaCO₃ and also from about 0.3% w/w to about 3.0% w/wGDL (see Table 1). Table 1 also provides an exemplary embodiment usingCaCO_(3/)GDL.

TABLE 1 Contrast Agent Gel Component % (w/w) % (w/w) Range CaCO₃ 0.50.1-1.0 GDL 1.5 0.3-3.0 Na-alginate 1.0 0.05-5.0  PEG-BNPs 9.5  5.0-15.0Water 88.0 75.0-95.0

Other suitable chelating gelation control reagents include:nitrilotriacetic acid (NTA), trans-1,2-diaminocylcohexanetetraaceticacid (DCTA), diethlyenetriaminepentaacetic acid (DTPA), andbis(aminoethyl)glycolether-N,N,N′,N′-tetraacetic acid (EGTA).

The radioopacity of the contrast agent can be controlled by adjustingthe concentration of capped nanoparticles in the gel solution. Suitably,the concentration of capped nanoparticles ranges from about 0.05 g/mL toabout 1 g/mL.

EXAMPLES Example 1

Materials and Methods

The nanoparticle core structure was produced using an arrestedprecipitation reaction. The foundational precipitation process is asfollows:

The BaSO₄(s) core particles are capped with the PEG polymer units thatserve to control the precipitation reaction. This process produces thePEG-capped BaSO₄ structure that is used to form and provide contrast inthe final gel structure.

The step-by-step synthesis for each component in the final X-raycontrast agent is given next. Amounts listed are for a typical synthesisand are not limiting on the disclosure. These amounts are only providedas typical examples.

Synthesis of PEG-BaSO₄ NPs (PBN)

Materials:

0.01 M Na₂SO₄ solution

0.01 M Ba(NO₃)2 solution

Polyethylene glycol (PEG-6000)

Procedure

Barium Sulfate synthesis can be carried out by the precipitationreaction: Ba(NO₃)₂ (aq)+Na₂SO₄ (aq)=BaSO₄ (ppt)+2NaNO₃ (aq)

To prevent bulk barium sulfate formation, capping agent (PEG-6000) wasadded during the precipitation process. First, 0.01 M Na₂SO₄ solutionwas prepared in a 100 mL volumetric flask using DI water. Na₂SO₄ wasrequired approximately 0.1420 gm. Then, 0.01 M Ba(NO₃)₂ solution wasprepared in another 100 mL volumetric flask using DI water. Ba(NO₃)₂ wasrequired approximately 0.2613 gm. 100 mL of 0.01 M Na₂SO₄ was taken in a500 mL beaker and approximately 2 mg of the PEG-6000 was added to thesolution with continuous stirring to completely dissolve the PEG in the0.01 M Na₂SO₄ solution. Then, previously prepared 0.01M Ba(NO₃)₂solution was added to the PEG-0.01M Na₂SO₄ solution dropwise withcontinuous stirring at a room temperature to get the PEG-BaSO₄ NPs.PEG-BaSO₄ NPs solution was dried in the oven at (80-100)° C. for about10-12 hours to obtain a PEG-BaSO₄ nanoparticle powder.

Preparation of the Gel

Materials required for the gelling process:

1% Na-Alginate

PBN powder

0.01 M EDTA

Procedure

First, 1 gm of Na-Alginate was added to the 100 mL DI water to prepare1% Na-Alginate. Then, 0.01M EDTA was prepared in a 100 mL volumetricflask from 0.1022 M stock solution. Approximately 0.1 gm of thePEG-BaSO₄ NPs powder was dissolved in the 0.5 mL 0.01M EDTA solution ina small vial. Micropipette was used to transfer the solution. Thesolution was then sonicated for about 5 minutes to dissolve the NPspowder in the 0.01M EDTA solution. Finally, 0.5 mL of the 1% Na-Alginatesolution was added to the PEG-BaSO₄-EDTA solution to make the gel.Gelling time was recorded using stopwatch. EDTA concentration can bevaried to control the gelling time.

Example 2 Synthesis of TEG@BaSO₄NPs:

Materials and Methods

2.61 M Na₂SO₄

0.61 M Ba(NO₃)2

44% Tetra ethylene Glycol (TEG)

Barium Sulfate synthesis was be carried out by the precipitationreaction:

Ba (NO₃)₂ (aq)+Na₂SO₄ (aq)→BaSO₄ (s)+2NaNO₃ (aq)

10 g of Na₂SO₄ was dissolved in 27 mL of DI water to make 2.61M Na₂SO₄.16 g of Ba(NO₃)₂ was dissolved in 100 mL of DI water to make 0.61 MBa(NO₃)₂. Both beakers were placed on hot plates set to high and mixedusing a glass rod. Once the Na₂SO₄ was completely dissolved, it wasmoved to another hot plate on a lower heat setting. In order to preventbulk barium sulfate formation, capping agent TEG was added during theprecipitation process. 100 mL of TEG was mixed with the dissolved 100 mLof Ba(NO₃)₂with a glass rod. A sonic dismembrator was used on the TEGand Ba(NO₃)₂ solution while adding the Na₂SO₄ dropwise with a Pasteurpipette. The sonic dismembrator was tuned so that it was highly pitchedand visibly mixing the solution (this was readjusted throughout theaddition). Drops were added closest to the sonic dismembrator probe at arate of 2 drops per second. After each full dispense of the pipette, thebeaker was moved so that the probe reached the edges in a circulartwisting motion and was then returned to the middle of the beaker. Ifthe Na₂SO₄ happened to crash out of solution, it was placed on the hotplate again and redissolved. Once all of the solutions were combinedunder the sonic dismembrator, a watch glass was placed on top of thebeaker holding the BNP's suspended in TEG. TEG@BaSO₄NPs solution wasdried in an oven at (80-100)° C. for about 10-12 hour to obtain theTEG@BaSO₄NPs powder.

TABLE 2 TEG@BaSO₄ NPs Tetra ethylene Glycol Ba(NO₃)₂ NaSO₄ (TEG) Water0.61M 2.61M 44% 300 mL

TABLE 3 Composition of TEG@BaSO₄ NPs and possible composition rangesMaterial Mass w/w % Composition^(a) Possible w/w % Range^(b) TEG 0.360.05-10   BaSO₄ 99.64 90-99.95 ^(a)The above described synthesisproduced a TEG@BaSO₄ NPs with this mass % composition (as estimated fromDSC-TGA). ^(b)Synthetic quantities can be adjusted to produce TEG@BaSO₄NPs with composition that falls within each range.

Example 3 Synthesis of TEG@CaCO₃ NPs:

Materials required for the synthesis process:

4.50 M CaCl₂

2.36 M Na₂CO₃

94% Tetra ethylene Glycol (TEG)

Calcium Carbonate Nanoparticles:

CaCl₂ (aq)+Na₂CO₃ (aq)→CaCO₃ (s)+2NaCl(aq)

0.5g of CaCl₂ was weighed into a glass vial and dissolved in 1 mL of DIwater by shaking to obtain a 4.5 M CaCl₂ solution. 0.5 g of Na₂CO₃ wasweighed into a glass vial and dissolved in 2 mL of DI water by shakingto obtain a 2.36 M Na₂CO₃ solution. 50 mL of TEG was mixed with thedissolved 1 mL CaCl₂ solution with a glass rod. A sonic dismembrator wasused on the TEG and CaCl₂ solution while adding the Na₂CO₃ dropwise witha Pasteur pipette. The sonic dismembrator was tuned so that it washighly pitched and visibly mixing the solution. Drops were added closestto the sonic dismembrator probe at a rate of 2 drops per second. Afteraddition was completed, the beaker was moved so that the probe reachedthe edges in a circular twisting motion and was then returned to themiddle of the beaker. Once all of the solutions were combined under thesonic dismembrator, a watch glass was placed on top of the beakerholding the calcium carbonate nanoparticles suspended in TEG.

TABLE 4 TEG@CaCO₃ NPs Tetra ethylene Glycol CaCl₂ Na₂CO₃ (TEG) Water4.50M 2.36M 94% 50 mL

TABLE 5 Compositions of TEG@ CaCO₃ NPs and possible composition rangesMaterial Mass w/w % Composition^(a) Possible w/w % Range^(b) TEG 23 5-30 CaCO₃ 67 70-95 ^(a)The above described synthesis produced a TEG@CaCO₃ NPs with this mass % composition (as estimated from DSC-TGA).^(b)Synthetic quantities can be adjusted to produce TEG@ CaCO₃ NPs withcomposition that falls within each range.

Example 4 Synthesis of 50 mL Nanocomposite Gel:

Materials required for the synthesis process:

0.80% Na-Alginate (0.40 gm in 50 mL)

2.50 gmTEG@BaSO₄NPs powder

0.50 gm Polyacrylic acid (PAA)

0.50 gm GDL (Glucono-δ-lactone)

0.20 gm TEG@CaCO₃

1.0 gm Na₂SO₄

0.80 gm of Na-Alginate was added to 100 mL DI water to prepare an

0.80% Na-Alginate solution. Approximately 2.50 gm of the TEG@BaSO₄ NPspowder was dispersed in 20 mL DI water. Then 0.20 gm of CaCO₃ was mixedwith the solution. Then 0.50 gm of Polyacrylic acid (PAA) added with thesolution. Then 1.0 gm of Na₂SO₄ was added to the solution. The solutionwas then sonicated for about 5 minutes to disperse the NPs powder in thesolution. Then 25 mL of the 0.80% Na-Alginate solution was added to theTEG@BaSO₄ solution. 0.50 gm of GDL (Glucono-δ-lactone) was dissolved in5.0 mL DI water. Finally, 5.0 mL of the GDL solution was mixed with the45.0 mL nanocomposite to initiate the crosslinking. Strong acid orstrong base may be added to adjust the pH to within the range 5.5-6.5 tooptimize crosslinking for most effective gelation. Gelling time wasrecorded using a stopwatch. GDL, PAA, Na-alginate and CaCO₃concentration was varied to control the gelling time.

TABLE 6 nanocomposite gel (50 mL) Glucono- Poly (acrylic Alginic Acid,δ-lactone acid sodium Gelation CaCO₃ BaSO₄ Sodium Salt (GDL) salt) (PAA)Na₂SO₄ time 0.20 gm 2.5 gm 0.40 gm 0.50 gm 0.50 gm 1.0 gm 30-35 min.

TABLE 7 Composition and composition ranges for the gel Mass w/w %Material Composition^(a) w/w % Range^(b) Notes Na-Alginate 0.025 0.01-0.1 TEG@BaSO₄ NPs 4.5    1-50 Polyacrylic acid 0.91  0.5-5 cGlucono-d-lactone 0.91 0.01-5 d TEG@CaCO₃ NPs 0.36 0.01-5 d Na₂SO₄ 1.8   0-10 ^(a)The above described synthesis produced a gel with this mass% composition (water not listed but makes up the remaining % mass).^(b)Synthetic quantities can be adjusted to produce a gel withcomposition that falls within each range. Water makes up any remainingmass.a c Polyvinyl alcohol can replace Polyacrylic acid - both should beincluded in coverage. d The gelling time for the gel synthesized above(with 0.36 w/w % CaCO₃ and 0.91 w/w % GDL) is 30 minutes. Adjustment ofthese compositional quantities can vary the gelling time from <1 minuteto 24 hours.

The present disclosure provides a contrast agent embedded in aninjectable polymeric matrix that can be delivered with a small-gaugeneedle that is minimally invasive. A novel nanoparticle structure isprovided that effectively reacts with aqueous alginate solutions toproduce stable gels and function as a contrast agent that isparticularly suitable for vascular imaging. The contrast agent has goodradiopacity, is inexpensive to produce, and is safe to handle. Thematerial can be introduced into the vasculature of animal systems andprovides good X-ray contrast. This provides a new method to image thefine vasculature of biological systems.

1. An injectable contrast agent comprising: a nanoparticle, the nanoparticle comprising a core comprising at least one of a barium sulfate (BaSO₄) core, a barium carbonate (BaCO₃) core, a calcium carbonate (CaCO₃) core and combinations thereof; and a tetra (ethylene glycol) capping agent; and a gel precursor solution.
 2. The contrast agent of claim 1, wherein the gel precursor solution is an alginate gel precursor solution.
 3. (canceled)
 4. The contrast agent of claim 1, wherein the amount of nanoparticle ranges from about 0.05 g/mL to about 1 g/mL.
 5. The contrast agent of claim 1, wherein the core diameter ranges from about 50 nm to about 500 nm.
 6. The contrast agent of claim 1, wherein the capping polymer ranges from about 0.05% (w/w) to about 30% (w/w).
 7. The contrast agent of claim 1, further comprising polyacrylic acid; polyvinyl alcohol, chitosan, and combinations thereof.
 8. The contrast agent of claim 1, further comprising a chelating gelation control reagent selected from the group consisting of ethylenediaminetetraacetic acid, glucono-δ-lactone in combination with calcium carbonate, nitrilotriacetic acid, trans-1,2-diaminocylcohexanetetraacetic acid, diethlyenetriaminepentaacetic acid, and bis(aminoethyl)glycolether-N,N,N′,N′-tetraacetic acid, and combinations thereof.
 9. A method for imaging vasculature of a subject, the method comprising: introducing into the subject's vasculature an injectable contrast agent, the injectable contrast agent comprising a nanoparticle, the nanoparticle comprising a core comprising at least one of a barium sulfate (BaSO₄) core, a barium carbonate (BaCO₃) core, a calcium carbonate (CaCO₃) core and combinations thereof; and a tetra (ethylene glycol) capping agent; and a gel precursor solution, wherein the nanoparticle is dispersed in the gel precursor solution; and imaging the subject using an X-ray technique.
 10. The method of claim 9, wherein the gel precursor solution is an alginate gel precursor solution.
 11. (canceled)
 12. The method of claim 9, wherein the amount of nanoparticle ranges from about 0.05 g/mL to about 1 g/mL.
 13. The method of claim 9, wherein the core diameter ranges from about 50 nm to about 500 nm.
 14. The method of claim 9, wherein the capping agent ranges from about 0.05% (w/w) to about 30% (w/w)
 15. The method of claim 9, wherein an amount of alginate in the alginate precursor solution ranges from about 0.5% (w/w) to about 5% (w/w).
 16. The method of claim 9, wherein the crosslinking gelation rate of the gel precursor solution ranges from about 10 seconds to about 48 hours at a temperature ranging from about 5° C. to about 40° C.
 17. The method of claim 9, wherein the injectable contrast agent further comprises polyacrylic acid; polyvinyl alcohol, chitosan, and combinations thereof.
 18. The method of claim 9, wherein the injectable contrast agent further comprises a chelating gelation control reagent selected from the group consisting of ethylenediaminetetraacetic acid, glucono-δ-lactone in combination with calcium carbonate, nitrilotriacetic acid, trans-1,2-diaminocylcohexanetetraacetic acid, diethlyenetriaminepentaacetic acid, and bis(aminoethyl)glycolether-N,N,N′,N′-tetraacetic acid, and combinations thereof. 